The present invention relates to a structure for the transport, commissioning and decommissioning of a deck of an offshore fixed oil production platform. Moreover, it relates to a method for the transport, commissioning and decommissioning of a deck of a fixed oil platform.
A known approach to oil production is to install above an oil field a fixed oil platform with a deck carrying production equipment and accommodation premises, in particular. A column anchored to the seabed supports the deck.
To date, there are two major methods for the transport, commissioning and decommissioning of a fixed oil platform deck.
The first method involves using barge-mounted lifting cranes for transfering the oil platform deck from the transport vessel to a superstructure comprising a supporting column. Although this method has been the most widely used to date, it has limitations.
The first limitation is effectively the capacity of the lifting cranes, which may require the deck to be fabricated in several units, thereby significantly increasing the fabrication cost of the oil platform deck and the cost of commissioning and decommissioning said deck.
The second limitation lies in the fact that this method imposes the need for a relatively long favorable time window to be able to perform this operation at sea under satisfactory conditions.
Without incurring a considerable increase in cost, this method is difficult to implement in areas in which time windows are relatively short, for example in the North Sea.
The second method involves commissioning the oil platform deck in a single unit by causing it to float over the supporting column. The deck is then positioned on this supporting column either by a ballasting/deballasting system or by a mechanical system.
In the case of a ballasting system, the oil platform deck is supported either by a floating support comprising, for example, a barge or pontoons or a U-shaped floating support, or by using a structure associated with this floating support.
In cases in which the superstructure can be ballasted or deballasted, deballasting of the floating support and ballasting of the superstructure is a known approach used to decommission an oil platform deck. The decommissioning operation can be performed relatively quickly because the superstructure offers a large ballasting capacity. In cases in which the superstructure is anchored to the seabed, only the deballasting capacity of the floating support can be used. The operation progresses slowly because this capacity is limited.
Systems using ballasting or deballasting have drawbacks due mainly to the fact that they require a complex structure in terms of chambers or pumps and very accurate control of chamber filling and emptying to maintain floating support stability during the operation. Operation speed depends on the ballasting and deballasting capacity of these floating support chambers, which is generally rather low inasmuch as the operation speed is often limited, especially when the superstructure is anchored to the seabed. Furthermore, maritime conditions must be favorable in order to perform such an operation in satisfactory conditions.
An alternative to the ballasting/deballasting system is to use a mechanical system to raise or lower the oil platform deck. These systems enable an oil platform deck commissioning or decommissioning operation to be performed faster than the abovementioned systems.
For this purpose, a known approach involves a system including two barges that support the oil platform deck using two swiveling structures. Furthermore, a cable and winch system is used to ensure system stability and control the raising and lowering of the oil platform deck.
Barge clearance is controlled by actuating these winches, thereby allowing the deck to be raised or lowered. However, the stability of this type of mechanical system is very insecure and it is frequently incompatible in relation to open sea use.
Another mechanical system comprises a rack and pinion system for raising or lowering the oil platform deck.
In general, mechanical systems used to date for commissioning and decommissioning an oil platform deck are faster than ballasting or deballasting systems, but they are dependent on maritime conditions, which makes them difficult to use in areas in which favorable time windows are relatively short.